1. Field of the Invention
The present invention relates to a production method of polyamide, more specifically, relates to a highly efficient production method of polyamide having excellent quality with minimized discoloration and deterioration and having stable properties.
2. Description of the Prior Art
Polyamide mainly constituted by m-xylylenediamine and adipic acid (hereinafter referred to as “nylon MXD6”) is excellent in mechanical and chemical properties and has been widely used as the raw materials for the production of molding materials and packaging materials. Since the melting point is as relatively high as about 240° C., nylon MXD6 is suitable for the production of composite packaging materials combined with a high melting resin such as poly(ethylene terephthalate). However, if combined with a resin having a melting point lower than that of nylon MXD6, for example, polyolefin such as polyethylene and polypropylene and polyamide such as nylon 6, the heat history on the low melting resin becomes higher than required to likely to cause a rapid heat deterioration thereof.
To overcome the above problem, it is effective to reduce the melting point of nylon MXD6. One method for attaining this is, for example, a method to introduce another kind of repeating unit which is capable of reducing the melting point of nylon MXD into the backbone of nylon MXD6 constituted by m-xylylenediamine/adipic acid repeating units. To reduce the melting point without deteriorating the mechanical and chemical properties of nylon MXD6, it has been proposed to randomly introduce m-xylylenediamine/aromatic dicarboxylic acid repeating units.
The method of introducing the m-xylylenediamine/aromatic dicarboxylic acid repeating unit into nylon MXD6 is proposed, for example, in Japanese Patent Application Laid-Open Nos. 57-200420 and 58-111829. In the proposed methods, polyamide is produced in a batch reactor by adding a diamine component dropwise into a molten dicarboxylic acid component such as adipic acid under atmospheric pressure while raising the temperature of the reaction system to proceed the polycondensation by removing the water released by condensation. This method is advantageous because the yield of polyamide in one batch is high and the apparatus cost is low.
The proposed methods may be practiced by first preparing a molten mixture of dicarboxylic acids by adding an aromatic dicarboxylic acid to a molten adipic acid or by melting a mixture of adipic acid and an aromatic dicarboxylic acid under heating, and then, adding m-xylylenediamine dropwise into the molten mixture of dicarboxylic acids.
Generally, the melting point of aromatic dicarboxylic acid is higher than that of straight-chain α,ω-aliphatic dicarboxylic acid. Therefore, there is a temperature range in which a mixture of dicarboxylic acids is in a suspension phase comprising a molten straight-chain α,ω-aliphatic dicarboxylic acid and a solid aromatic dicarboxylic acid not dissolved in the molten straight-chain α,ω-aliphatic dicarboxylic acid. For example, since adipic acid melts at 170° C. but the aromatic dicarboxylic acid generally remains solid without dissolved in the molten straight-chain α,ω-aliphatic dicarboxylic acid at this temperature, a mixture thereof is in a suspension phase. If a large amount of m-xylylenediamine is added dropwise into a mixture in the suspension phase, the added m-xylylenediamine reacts with the molten adipic acid in preference to the solid aromatic dicarboxylic acid. Therefore, the reaction between m-xylylenediamine with the aromatic dicarboxylic acid does not proceed sufficiently. In addition, the m-xylylenediamine/aromatic dicarboxylic acid repeating units are not randomly introduced in the backbone, but the m-xylylenediamine/adipic acid repeating units and the m-xylylenediamine/aromatic dicarboxylic acid repeating units respectively form distinct blocks, thereby likely to result in the failure of reducing the melting point of polyamide as well as likely to make the mechanical and chemical properties unstable.
To produce a polyamide constituted by randomly arranged m-xylylenediamine/adipic acid repeating units and m-xylylenediamine/aromatic dicarboxylic acid repeating units by enhancing the reaction with the aromatic dicarboxylic acid, proposed is a method of using a nylon salt or its aqueous solution. For example, an aqueous solution of a mixture of a m-xylylenediamine/adipic acid salt and a m-xylylenediamine/aromatic dicarboxylic acid salt which are prepared in advance is heated under pressure in a reaction vessel. Then, the polymerization is proceeded in homogeneous condition while preventing the escape of the diamine component. After fixing the diamine component, the pressure of the reaction system is gradually reduced finally to atmospheric pressure or reduced pressure by releasing the water vapor to complete the polymerization.
However, the method of using the nylon slat as the starting material involves several problems. For example, an about 50% aqueous solution of nylon salt is generally used as the starting material in this method. Therefore, a highly pressure-resistant apparatus is required in the initial stage of polymerization to prevent the solvent water from escaping by distillation. In addition, a large amount of the solvent water and the water released by condensation should be finally removed. During the removal of the water, several troubles are caused, for example, expansion by foaming, solidification of polyamide due to the latent heat of water vaporization, adhesion of polyamide onto the inner wall of the reaction vessel due the large change of reaction liquid level, and heat deterioration of the adhered polyamide. Therefore, it is required to take precautions against these troubles. In addition, the method involves several technical problems because, for example, a large amount of heat energy is needed to remove a large amount of water. Particularly, the method cannot be said as efficient, because the batch yield of polyamide is low. These problems can be somewhat solved by the polymerization methods using the nylon salt described in Japanese Patent Publication Nos. 33-15700 and 43-22874. However, these methods are also cannot be said as efficient, because the steps for isolating and purifying the nylon salt are necessary.